What are the general causes for plants dying off? I have 2 tanks, and the same exact plants in both tanks. Water and treatments are the same in both tanks, yet in one tank, the plants are dying and in the other they are not. The only difference is that in the tank where the plants are dying, my fish suffers from constipation. He is fasted often as a result. In the other, my fish is a pooping machine and has NEVER had a problem with constipation. In the dying plant tank, after a week (I change the water weekly), there is hardly any poop to suck out. In the healthy plant tank, there is tons and tons of waste. They are both cycled tanks and readings are normal.

Could the lack of fish waste be a factor here? And if so, would fertilizers help? Which fertilizers are good to use?

Huh, that's kinda interesting. Is the lighting the same for both tanks? If so and everything else is the same, it sounds like it could be that the fish with constipation just doesn't give off enough nutrients for the plants to thrive.

You could try ferts to see if it helps... A really good one is Flourish Comprehensive. It's a bit more expensive than the other fertilizers out there, but like most Seachem products, it's super concentrated and will last a really long time.

It is the same lighting in both tanks. They are the exact same tanks, they have an LED lighting set up, and there (unfortunately) no natural sunlight in that room. Rearranging so that there natural lighting, is not an option - our windows face north-west, and since that is our bedroom and there is annoying lamp right outside, we have darkening shades over the window that don't open. The only other window is in the Living room and it also face North-West and is right next to the front door - and in the summer, that window can get up to 90 or higher temps, so we have to put the air conditioner in that window.
But like I said, the other tank's plants are thriving with no natural lighting. A little too well in fact, I've had to trim them down multiple times just so she can swim.
I will buy some fertilizers and see if that helps.